The present invention relates to electrophotographic photoconductors having improved electrical characteristics in a positive charging process. The present invention further relates to electrophotographic apparatus using the aforementioned electrophotographic photoconductor.
In recent years, numerous electrophotographic photoconductors using organic photosensitive materials have been proposed and put to practical use. These conventional electrophotographic photoconductors had certain advantages over the prior photoconductors, such as freedom from environmental pollution, low cost, and flexibility of designing electrophotographic characteristics provided by wide variety in material selection.
A photosensitive layer of an organic electrophotographic photoconductor principally consists of a layer having organic photosensitive material dispersed in a resin material. Many types of photosensitive layer structures have been proposed, including laminate structures and single-layer structures. A laminate structure consists of a charge generation layer, containing a charge generation substance, dispersed in a resin, and a charge transport layer, containing charge transport substance, dispersed in a resin. A single-layer structure consists of a single-layer containing both a charge generation substance and a charge transport substance dispersed in a resin.
In conventional functional-separated type photoconductor, a photosensitive layer consists of a charge transport layer and a charge generation layer. The charge transport layer is laminated on a charge generation layer. This type of photoconductor is widely used because of its excellent photoconductor characteristics and durability. The charge transport layer of the function-separated laminate type photoconductor generally uses a hole transport substance. Therefore, the type of photoconductor is utilized in a negative-charging process. Negative polar corona discharge in the negative-charging process is less stable and generates larger amount of ozone as compared to a positive polar corona discharge. This raises problems such as undesirable effects to the photoconductor and hazardous influence to the operation environment.
These problems are effectively solved by an organic electrophotographic photoconductor which can be used in a positive-charging process. Thus, a positive-charging type photoconductor exhibiting high sensitivity is presently desired. Numerous photosensitive layers of the positive-charging type photoconductor have been proposed. A function-separated type photoconductor having a charge generation layer laminated on a hole transport layer has been proposed. Furthermore, a function-separated type photoconductor having an electron transport layer laminated on a charge generation layer has also been proposed. Additionally, a single-layer type photoconductor having a charge generation substance and charge transport substance contained in the same layer has been proposed. However, most of these conventional photoconductors are inferior in electrical characteristics, such as sensitivity, as compared to photosensitive layers of negative-charging function-separated photoconductors.
In the meantime, many kinds of electron transport substances and electrophotographic photoconductors using these electron transport substances have been proposed. Such substances and photoconductors have recently received extensive attention. The references include Japanese Unexamined Patent Application Publication (KOKAI) Nos. H1-206349 and H4-360148, Denshishashin gakkaishi (Electrophotography) vol. 30, p. 266-273 (1991), Japanese Unexamined Patent Application Publication (KOKAI) Nos. H3-290666, H5-92936, Preprint p.207-210 of Pan-Pacific Imaging Conference/Japan Hardcopy ""98 Jul. 15-17, 1998, J A Hall, Tokyo, Japan, Japanese Unexamined Patent Application Publication (KOKAI) No. H9-151157, Proceedings p.21-24 of Japan Hardcopy ""97, Jul. 9-11, 1997, J A Hall, Tokyo, Japan, Japanese Unexamined Patent Application Publication (KOKAI) Nos. H5-279582 and H7-179775, Proceedings p.173-176 of Japan Hardcopy ""92, Jul. 6-8, 1992, J A Hall, Tokyo, Japan, and Japanese Unexamined Patent Application Publication (KOKAI) No. H10-73937. A photoconductor having a single-layer photosensitive layer containing an electron transport substance drew attention as possessing high sensitivity. Some of these photoconductors have been put into practice.
Conventional single-layer type electrophotographic photoconductors containing electron transport substance, as disclosed in the above-cited references, exhibit initial electrical characteristics, such as sensitivity and residual potential, better than photoconductors without electron transport substance. There still remains, however, the problem that repeated use of these conventional single-layer type electrophotographic photoconductors deteriorates the electrical characteristics. This problem is remarkable when used in an environment with high temperature and high humidity or with low temperature and low humidity.
Biphenyl derivatives are used as a plasticizer for plastics. Some applications of biphenyl derivatives in an electrophotographic photoconductor, as a plasticizer for relaxation of residual stress, have been proposed in Japanese Unexamined Patent Application Publications (KOKAI) Nos. H3-134670, H6-75394, H6-148914, and H7-92702. Further applications of biphenyl derivatives, as a deactivating agent contemplating stability in repeated use by preventing dimerization or dissociation of certain charge transport substances, have been proposed in Japanese Unexamined Patent Application Publications (KOKAI) Nos. H3-75754, H3-75755, and H3-75756. An additional application of biphenyl derivatives, as an agent for increasing electrostatic force and van der Vaals"" force contemplating high image density, is proposed in Japanese Unexamined Patent Application Publication (KOKAI) No. H5-273771. An application of biphenyl derivatives in combination with an undercoat layer containing inorganic pigment for improving resistance to NOx, contemplating stability in repeated use, is disclosed in Japanese Unexamined Patent Application Publication (KOKAI) No. H7-306540. Finally, an application containing the biphenyl derivative in a charge generating layer to improve adhesivity of the charge generating layer to a charge transport layer, an intermediate layer, or a substrate is proposed in Japanese Unexamined Patent Application Publication (KOKAI) No. H10-268532.
It is therefore an object of the present invention to provide an electrophotographic photoconductor having a single-layer photosensitive layer containing electron transport substance that is free from the above faults.
It is a further object of the present invention to provide an electrophotographic photoconductor that exhibits excellent electrical characteristics in a positive charging process and excellent stability in repeated use.
It is also an object of the present invention to provide an electrophotographic apparatus equipped with such a photoconductor.
Briefly stated, the present invention provides a biphenyl derivative included in a photosensitive layer of an electrophotographic photoconductor. The electrophotographic photoconductor includes a single-layer photosensitive layer coated on a conductive substrate. The photosensitive layer includes a resin binder, a charge generation substance, a hole transport substance, and an electron transport substance. An electrophotographic apparatus using such an electrophotographic photoconductor has excellent electrical characteristics in a positive charging system and is stable in repeated use.
According to an embodiment of the present invention, there is provided an electrophotographic photoconductor comprising: a conductive substrate; a single-layer photosensitive layer disposed on the conductive substrate; the single-layer photosensitive layer including a resin binder, a charge generation substance, a hole transport substance, and an electron transport substance; and the single-layer photosensitive layer contains a biphenyl derivative.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.